This invention relates in general to means and methods of checking the integrity of individual electrical connections between an array of photosensors adapted to produce an electric current in response to photon impingement thereon and a corresponding array of capacitive storage cells, and in particular to hybrid devices in which two such arrays are intended to be electrically connected by flip-chip bonding processes, each of said storage cells having an effective parasitic resistance of much greater magnitude than its corresponding photosensor.
Flip-chip hybrid bonding is a vertical interconnect process for mating two or more microelectronic chips. Electrical connections are made by means of conductive solder bumps or bonding points between the chips. In the process, a first chip with an array of solder, or other bonding medium, bumps formed thereon is flipped over (flip-chip), aligned to, and pressed onto a corresponding array of bonding pads or bumps on the second chip. The compression mates the solder bumps to the bonding pads establishing both mechanical and electrical connections between the two chips.
A major problem with this technology is the monitoring of the typically large number of bump/bonding pad interconnects which are formed. The interconnects can number in the thousands. Typically, the solder bumps are not all of exactly the same height so different bumps will bridge the gap between the two chips at different times during the hybridization. If there is insufficient compression, then the shorter bumps will not make contact with their associated bonding pads, thus leaving an open circuit when a closed circuit was intended. If there is too much compression, then the taller bumps may become so deformed that they expand laterally and short to their nearest neighbors.
Normal visual inspection methods are an impractical means of monitoring the interconnects because of their number and because most of the interconnects are hidden from view by their surrounding neighbors. The conventional process for checking the interconnect status is to stop the hybridization, mount the device, bring it to its operating temperature, and run it. If a good electrical connection has been made between the two chips, then the circuit associated with each interconnect will respond properly. If additional compression is required, the device must be reinserted into the hybrid aligner-bonder, realigned, recompressed and retested until the required number of interconnects are properly made.
This invention provides a means of obtaining an immediate feedback as to the status of all of the bump/bonding pad interconnects. Such information is useful for determining how well the hybridization is proceeding as well as when it is complete. Early monitoring in hybridization provides important information on how well the alignment of the bumps and pads was performed. For example, if the two chips are not properly coplanarized, then one corner of the chips will undergo hybridization long before the opposing corner does. For hybrid devices which are normally operated at cryogenic temperatures, room temperature monitoring, as provided by this invention, also eliminates the time and need to cool the devices to their operating temperatures for intermediate testing. Finally, since the interconnects of the chips can be tested while the chips are still mounted in the hybridization chucks, the potential for damaging the chips by mounting and dismounting is minimized or altogether eliminated.
Other advantages and attributes of this invention will be discussed or will otherwise be readily discernible upon a reading of the text hereinafter.